Facile Physical Route to Highly Crystalline Graphene

A physical route is proposed to obtain highly crystalline graphene sheets with minimal oxygen content similar to the precursor graphite. The functional graphene sheets obtained from graphite oxide by low temperature thermal exfoliation are annealed at high temperature (1900 °C) in a vacuum (10^?6 torr). The D band intensity in Raman spectroscopy is reduced significantly, while the G band intensity is recovered, similar to the level of precursor graphite. No appreciable oxygen content is observed from X‐ray photoelectron spectroscopy and an electrical conductivity of ～56 500 S m^?1 is obtained, comparable to 100 900 S m^?1 of the precursor graphite.     

Strongly Anisotropic Thermal Conductivity of Free‐Standing Reduced Graphene Oxide Films Annealed at High Temperature

Thermal conductivity of free‐standing reduced graphene oxide films subjected to a high‐temperature treatment of up to 1000 °C is investigated. It is found that the high‐temperature annealing dramatically increases the in‐plane thermal conductivity, K, of the films from ≈3 to ≈61 W m^?1 K^?1 at room temperature. The cross‐plane thermal conductivity, K_⊥, reveals an interesting opposite trend of decreasing to a very small value of ≈0.09 W m^?1 K^?1 in the reduced graphene oxide films annealed at 1000 °C. The obtained films demonstrate an exceptionally strong anisotropy of the thermal conductivity, K/K_⊥ ≈ 675, which is substantially larger even than in the high‐quality graphite. The electrical resistivity of the annealed films reduces to 1–19 Ω □^?1. The observed modifications of the in‐plane and cross‐plane thermal conductivity components resulting in an unusual K/K_⊥ anisotropy are explained theoretically. The theoretical analysis suggests that K can reach as high as ≈500 W m^?1 K^?1 with the increase in the sp² domain size and further reduction of the oxygen content. The strongly anisotropic heat conduction properties of these films can be useful for applications in thermal management.     

Thermally Conductive Reduced Graphene Oxide Thin Films for Extreme Temperature Sensors

Reduced graphene oxide (RGO) films are promising in applications ranging from electronics to flexible sensors. Though high electrical and thermal conductivities have been reported for RGO films, existing thermal conductivity data for RGO films show large variations from 30 to 2600 W m^?1 K^?1. Further, there is a lack of data at low temperatures (<300 K), which is critical for the understanding of thermal transport mechanisms. In this work, a temperature‐dependent study of thermal (10–300 K) and electrical (10–3000 K) transport in annealed RGO films indicates the potential application of RGO films for sensing temperatures across an extremely wide range. The room‐temperature thermal conductivity increases significantly from 46.1 to 118.7 W m^?1 K^?1 with increasing annealing temperature from 1000 to 3000 K with a corresponding increase in the electrical conductivity from 5.2 to 1481.0 S cm^?1. In addition, films reduced at 3000 K are promising for sensing extreme temperatures as demonstrated through the measured electrical resistivity from 10 to 3000 K. Sensors based on RGO films are advantageous over conventional temperature sensors due to the wide temperature range and flexibility. Thus, this material is useful in many applications including flexible electronics and thermal management systems.     

Electric‐Field‐Assisted Growth of Vertical Graphene Arrays and the Application in Thermal Interface Materials

Owing to the development of electronic devices moving toward high power density, miniaturization, and multifunction, research on thermal interface materials (TIMs) is become increasingly significant. Graphene is regarded as the most promising thermal management material owing to its ultrahigh in‐plane thermal conductivity. However, the fabrication of high‐quality vertical graphene (VG) arrays and their utilization in TIMs still remains a big challenge. In this study, a rational approach is developed for growing VG arrays using an alcohol‐based electric‐field‐assisted plasma enhanced chemical vapor deposition. Alcohol‐based carbon sources are used to produce hydroxyl radicals to increase the growth rate and reduce the formation of defects. A vertical electric field is used to align the graphene sheets. Using this method, high‐quality and vertically aligned graphene with a height of 18.7 µm is obtained under an electric field of 30 V cm^?1. TIMs constructed with the VG arrays exhibit a high vertical thermal conductivity of 53.5 W m^?1 K^?1 and a low contact thermal resistance of 11.8 K mm² W^?1, indicating their significant potential for applications in heat dissipation technologies.     

An Ultralight Graphene Honeycomb Sandwich for Stretchable Light‐Emitting Displays

Materials that can simultaneously achieve excellent electrical conductivity and stretchability are essential components for the next‐generation stretchable electronics. Herein, a highly conductive and stretchable graphene honeycomb (GHC) sandwich consisting of a porous GHC/polydimethylsiloxane (PDMS) composite core and ultrahigh‐density graphene foam/PDMS composite face sheets is rationally assembled. The GHCs prepared by a 3D printing technique possess a long‐range ordered hexagonal porous structure with an ultralow density of 3.25 mg cm^?3 and deliver an excellent electrical conductivity of 72 S m^?1. To the best of authors' knowledge, they are the lightest honeycomb structure reported in open literature. The highly porous GHC core endows the sandwich with an ultralow resistance change over large external strains of 60% with excellent structural durability. A stretchable light‐emitting display is demonstrated using the sandwich as the electric circuit and light‐emitting diodes as the pixels, which exhibits reliable lighting performance under different loading conditions.     

Highly Conductive Porous Graphene/Ceramic Composites for Heat Transfer and Thermal Energy Storage

A novel architecture of 3D graphene growth on porous Al₂O₃ ceramics is proposed for thermal management using ambient pressure chemical vapor deposition. The formation mechanism of graphene is attributed to the carbothermic reduction occurring at the Al₂O₃ surface to initialize the nucleation and growth of graphene. The graphene films are coated on insulating anodic aluminum oxide (AAO) templates and porous Al₂O₃ ceramic substrates. The graphene coated AAO possesses one‐dimensional isolated graphene tubes, which can act as the media for directional thermal transport. The graphene/Al₂O₃ composite (G‐Al₂O₃) contains an interconnected macroporous graphene framework with an extremely low sheet electrical resistance down to 0.11 Ω sq^?1 and thermal conductivity with 8.28 W m^?1 K^?1. The G‐Al₂O₃ provides enormous conductive pathways for electronic and heat transfer, suitable for application as heat sinks. Such a porous composite is also attractive as a highly thermally conductive reservoir to hold phase change materials (stearic acid) for thermal energy storage. This work displays the great potential of CVD direct growth of graphene on dielectric porous substrates for thermal conduction and electronic applications.     

Thermal Properties of the Binary‐Filler Hybrid Composites with Graphene and Copper Nanoparticles

The thermal properties of epoxy‐based binary composites comprised of graphene and copper nanoparticles are reported. It is found that the “synergistic” filler effect, revealed as a strong enhancement of the thermal conductivity of composites with the size‐dissimilar fillers, has a well‐defined filler loading threshold. The thermal conductivity of composites with a moderate graphene concentration of f_g = 15 wt% exhibits an abrupt increase as the loading of copper nanoparticles approaches f_Cu ≈ 40 wt%, followed by saturation. The effect is attributed to intercalation of spherical copper nanoparticles between the large graphene flakes, resulting in formation of the highly thermally conductive percolation network. In contrast, in composites with a high graphene concentration, f_g = 40 wt%, the thermal conductivity increases linearly with addition of copper nanoparticles. A thermal conductivity of 13.5 ± 1.6 Wm^?1K^?1 is achieved in composites with binary fillers of f_g = 40 wt% and f_Cu = 35 wt%. It has also been demonstrated that the thermal percolation can occur prior to electrical percolation even in composites with electrically conductive fillers. The obtained results shed light on the interaction between graphene fillers and copper nanoparticles in the composites and demonstrate potential of such hybrid epoxy composites for practical applications in thermal interface materials and adhesives.     

Emulsifier‐Free Graphene Dispersions with High Graphene Content for Printed Electronics and Freestanding Graphene Films

A novel and highly versatile synthetic route for the production of functionalized graphene dispersions in water, acetone, and isopropanol (IPA), which exhibit long‐term stability and are easy to scale up, is reported. Both graphene functionalization (wherein the oxygen content can be varied from 4 to 16 wt%) and dispersion are achieved by the thermal reduction of graphite oxide, followed by a high‐pressure homogenization (HPH) process. For the first time, binders, dispersing agents, and reducing agents are not required to produce either dilute or highly concentrated dispersions of single graphene sheets with a graphene content of up to 15 g L^?1. High graphene content is essential for the successful printing of graphene dispersions by 3D microextrusion. Free‐standing graphene films and micropatterned graphene materials are successfully prepared using this method. Due to the absence of toxic reducing agents, the graphene exhibits no cytotoxicity and is biocompatible. Furthermore, the electrical conductivity of graphene is significantly improved by the absence of binders. Flexible microarrays can be printed on different substrates, producing microarrays that are mechanically stable and can be bent several times without affecting electrical conductivity.     

Graphene Oxide Based Transparent Resins For Accurate 3D Printing of Conductive Materials

Digital Light Processing (DLP) allows the fast realization of 3D objects with high spatial resolution. However, DLP is limited to transparent resins, and therefore not well suited for printing electrically conductive materials. Manufacturing conductive materials will significantly broaden the spectrum of applications of the DLP technology. But conductive metals or carbon-based fillers absorb and scatter light; inhibiting thereby photopolymerization, and lowering resolution. In this study, UV transparent liquid crystal graphene oxide (GO) is used as precursor for generating in situ conductive particles. The GO materials are added to a photopolymerizable resin via an original solvent exchange process. By contrast to earlier contributions, the absence of drying during the all process allows the GO material to be transferred as monolayers to limit UV scattering. The absence of UV scattering and absorption allows for fast and high-resolution 3D printing. The chosen resin sustain high temperature to enable an in situ efficient thermal reduction of GO into reduced graphene oxide (rGO) that is electrically conductive. The rGO particles form percolated networks with conductivities up to 1.2 × 10⁻² S m⁻¹. The present method appears therefore as a way to reconcile the DLP technology with the manufacturing of 3D electrically conductive objects.     

Novel Cu Nanowires/Graphene as the Back Contact for CdTe Solar Cells

Cu‐nanowire‐doped graphene (Cu NWs/graphene) is successfully incorporated as the back contact in thin‐film CdTe solar cells. 1D, single‐crystal Cu nanowires (NWs) are prepared by a hydrothermal method at 160 °C and 3D, highly crystalline graphene is obtained by ambient‐pressure CVD at 1000 °C. The Cu NWs/graphene back contact is obtained from fully mixing the Cu nanowires and graphene with poly(vinylidene fluoride) (PVDF) and N‐methyl pyrrolidinone (NMP), and then annealing at 185 °C for solidification. The back contact possesses a high electrical conductivity of 16.7 S cm^?1 and a carrier mobility of 16.2 cm² V^?1 s^?1. The efficiency of solar cells with Cu NWs/graphene achieved is up to 12.1%, higher than that of cells with traditional back contacts using Cu‐particle‐doped graphite (10.5%) or Cu thin films (9.1%). This indicates that the Cu NWs/graphene back contact improves the hole collection ability of CdTe cells due to the percolating network, with the super‐high aspect ratio of the Cu nanowires offering enormous electrical transport routes to connect the individual graphene sheets. The cells with Cu NWs/graphene also exhibit an excellent thermal stability, because they can supply an active Cu diffusion source to form an stable intermediate layer of CuTe between the CdTe layer and the back contact.     

Multilayer‐Folded Graphene Ribbon Film with Ultrahigh Areal Capacitance and High Rate Performance for Compressible Supercapacitors

Limited by 2D geometric morphology and low bulk packing density, developing graphene‐based flexible/compressible supercapacitors with high specific capacitances (gravimetric/volumetric/areal), especially at high rates, is an outstanding challenge. Here, a strategy for the synthesis of free‐standing graphene ribbon films (GRFs) for high‐performance flexible and compressible supercapacitors through blade‐coating of interconnected graphene oxide ribbons and a subsequent thermal treatment process is reported. With an ultrahigh mass loading of 21 mg cm^?2, large ion‐accessible surface area, efficient electron and ion transport pathways as well as high packing density, the compressed multilayer‐folded GRF films (F‐GRF) exhibit ultrahigh areal capacitance of 6.7 F cm^?2 at 5 mA cm^?2, high gravimetric/volumetric capacitances (318 F g^?1, 293 F cm^?3), and high rate performance (3.9 F cm^?2 at 105 mA cm^?2), as well as excellent cycling stability (109% of capacitance retention after 40 000 cycles). Furthermore, the assembled F‐GRF symmetric supercapacitor with compressible and flexible characteristics, can deliver an ultrahigh areal energy density of 0.52 mWh cm^?2 in aqueous electrolyte, almost two times higher than the values obtained from symmetric supercapacitors with comparable dimensions.     

Stress Controllability in Thermal and Electrical Conductivity of 3D Elastic Graphene‐Crosslinked Carbon Nanotube Sponge/Polyimide Nanocomposite

Stress controllability in thermal and electrical conductivity is important for flexible piezoresistive devices. Due to the strength‐elasticity trade‐off, comprehensive investigation of stress‐controllable conduction in elastic high‐modulus polymers is challenging. Here presented is a 3D elastic graphene‐crosslinked carbon nanotube sponge/polyimide (G_w‐CNT/PI) nanocomposite. Graphene welding at the junction enables both phonon and electron transfer as well as avoids interfacial slippage during cyclic compression. The uniform G_w‐CNT/PI comprising a high‐modulus PI deposited on a porous templated network combines stress‐controllable thermal/electrical conductivity and cyclic elastic deformation. The uniform composites show different variation trends controlled by the porosity due to different phonon and electron conduction mechanisms. A relatively high k (3.24 W m^?1 K^?1, 1620% higher than PI) and suitable compressibility (16.5% under 1 MPa compression) enables the application of the composite in flexible elastic thermal interface conductors, which is further analyzed by finite element simulations. The interconnected network favors a high stress‐sensitive electrical conductivity (sensitivity, 973% at 9.6% strain). Thus, the G_w‐CNT/PI composite can be an important candidate material for piezoresistive sensors upon porosity optimization based on stress‐controllable thermal or electrical conductivity. The results provide insights toward controlling the stress‐induced thermal/electrical conductivities of 3D interconnected templated composite networks for piezoresistive conductors or sensors.     

Highly Conductive Carbon Nanotube‐Graphene Hybrid Yarn

An efficient procedure for the fabrication of highly conductive carbon nanotube/graphene hybrid yarns has been developed. To start, arrays of vertically aligned multi‐walled carbon nanotubes (MWNT) are converted into indefinitely long MWNT sheets by drawing. Graphene flakes are then deposited onto the MWNT sheets by electrospinning to form a composite structure that is transformed into yarn filaments by twisting. The process is scalable for yarn fabrication on an industrial scale. Prepared materials are characterized by electron microscopy, electrical, mechanical, and electrochemical measurements. It is found that the electrical conductivity of the composite MWNT‐graphene yarns is over 900 S/cm. This value is 400% and 1250% higher than electrical conductivity of pristine MWNT yarns or graphene paper, respectively. The increase in conductivity is asssociated with the increase of the density of states near the Fermi level by a factor of 100 and a decrease in the hopping distance by an order of magnitude induced by grapene flakes. It is found also that the MWNT‐graphene yarn has a strong electrochemical response with specific capacitance in excess of 111 Fg^?1. This value is 425% higher than the capacitance of pristine MWNT yarn. Such substantial improvements of key properties of the hybrid material can be associated with the synergy of MWNT and graphene layers in the yarn structure. Prepared hybrid yarns can benefit such applications as high‐performance supercapacitors, batteries, high current capable cables, and artificial muscles.     

Nitrogen‐Doped Graphene Ribbon Assembled Core–Sheath MnO@Graphene Scrolls as Hierarchically Ordered 3D Porous Electrodes for Fast and Durable Lithium Storage

Graphene scroll is an emerging 1D tubular form of graphitic carbon that has potential applications in electrochemical energy storage. However, it still remains a challenge to composite graphene scrolls with other nanomaterials for building advanced electrode configuration with fast and durable lithium storage properties. Here, a transition‐metal‐oxide‐based hierarchically ordered 3D porous electrode is designed based on assembling 1D core–sheath MnO@N‐doped graphene scrolls with 2D N‐doped graphene ribbons. In the resulting architecture, porous MnO nanowires confined in tubular graphene scrolls are mechanically isolated but electronically well‐connected, while the interwoven graphene ribbons offer continuous conductive paths for electron transfer in all directions. Moreover, the elastic graphene scrolls together with enough internal voids are able to accommodate the volume expansion of the enclosed MnO. Because of these merits, the as‐built electrode manifests ultrahigh rate capability (349 mAh g^?1 at 8.0 A g^?1; 205 mAh g^?1 at 15.0 A g^?1) and robust cycling stability (812 mAh g^?1 remaining after 1000 cycles at 2.0 A g^?1) and is the most efficient MnO‐based anode ever reported for lithium‐ion batteries. This unique multidimensional and hierarchically ordered structure design is believed to hold great potential in generalizable synthesis of graphene scrolls composited with oxide nanowires for mutifuctional energy storage.     

Dual Electrostatic Assembly of Graphene Encapsulated Nanosheet‐Assembled ZnO‐Mn‐C Hollow Microspheres as a Lithium Ion Battery Anode

Graphene encapsulated nanosheet‐assembled ZnO‐Mn‐C hierarchical hollow microspheres are produced through a simple yet effective dual electrostatic assembly strategy, followed by a heating treatment in inert atmosphere. The modification of graphene sheets, metal Mn, and in situ carbon leads to form 3D interconnected conductive framework as electron highways. The hollow structure and the open configuration of hierarchical microspheres guarantee good structural stability and rapid ionic transport. More importantly, according to the density functional theory calculations, the oxygen vacancies in the hierarchical microspheres would cause an imbalanced charge distribution and thus the formation of local in‐plane electric fields around oxygen vacancy sites, which is beneficial for the ionic/electronic transport during cycling. Due to this multiscale coordinated design, the as‐fabricated graphene encapsulated nanosheet‐assembled ZnO‐Mn‐C hierarchical hollow microspheres demonstrate good lithium storage properties in terms of high reversible capacity (1094 mA h g^?1 at 100 mA g^?1), outstanding high‐rate long‐term cycling stability (843 mA h g^?1 after 1000 cycles at 2000 mA g^?1), and remarkable rate capability (422 mA h g^?1 after total 1600 cycles at 5000 mA g^?1).     

Simultaneous Electrochemical Dual‐Electrode Exfoliation of Graphite toward Scalable Production of High‐Quality Graphene

Developing scalable methods to produce large quantities of high‐quality and solution‐processable graphene is essential to bridge the gap between laboratory study and commercial applications. Here an efficient electrochemical dual‐electrode exfoliation approach is developed, which combines simultaneous anodic and cathodic exfoliation of graphite. Newly designed sandwich‐structured graphite electrodes which are wrapped in a confined space with porous metal mesh serve as both electrodes, enabling a sufficient ionic intercalation. Mechanism studies reveal that the combination of electrochemical intercalation with subsequent thermal decomposition results in drastic expansion of graphite toward high‐efficiency production of graphene with high quality. By precisely controlling the intercalation chemistry, the two‐step approach leads to graphene with outstanding yields (85% and 48% for cathode and anode, respectively) comprising few‐layer graphene (1–3 layers, >70%), ultralow defects (I_D/I_G < 0.08), and high production rate (exceeding 25 g h^?1). Moreover, its excellent electrical conductivity (>3 × 10⁴ S m^?1) and great solution dispersibility in N‐methyl pyrrolidone (10 mg mL^?1) enable the fabrication of highly conductive (11 Ω sq^?1) and flexible graphene films by inkjet printing. This simple and efficient exfoliation approach will facilitate the development of large‐scale production of high‐quality graphene and holds great promise for its wide application.     

Oxygen‐Free Highly Conductive Graphene Papers

Graphene papers have a potential to overcome the gap from nanoscale graphene to real macroscale applications of graphene. A unique process for preparation of highly conductive graphene thin paper by means of Ar⁺ ion irradiation of graphene oxide (GO) papers, with carbon/oxygen ratio reduced to 100:1, is presented. The composition of graphene paper in terms of carbon/oxygen ratio and in terms of types of individual oxygen‐containing groups is monitored throughout the process. Angle‐resolved high resolution X‐ray photoelectron spectroscopy helps to investigate the depth profile of carbon and oxygen within reduced GO paper. C/O ratios over 100 on the surface and 40 in bulk material are observed. In order to bring insight to the processes of oxygen removal from GO paper by low energy Ar⁺ ion bombardment, the gases released during the irradiation are analyzed by mass spectroscopy. It is proven that Ar⁺ ion beam can be applied as a technique for fabrication of highly reduced graphene papers with high conductivities. Such highly conductive graphene papers have great potential to be used in application for construction of microelectronic and sensor devices.     

Micro‐Supercapacitors Based on Interdigital Electrodes of Reduced Graphene Oxide and Carbon Nanotube Composites with Ultrahigh Power Handling Performance

A novel method for fabricating micro‐patterned interdigitated electrodes based on reduced graphene oxide (rGO) and carbon nanotube (CNT) composites for ultra‐high power handling micro‐supercapacitor application is reported. The binder‐free microelectrodes were developed by combining electrostatic spray deposition (ESD) and photolithography lift‐off methods. Without typically used thermal or chemical reduction, GO sheets are readily reduced to rGO during the ESD deposition. Electrochemical measurements show that the in‐plane interdigital design of the microelectrodes is effective in increasing accessibility of electrolyte ions in‐between stacked rGO sheets through an electro‐activation process. Addition of CNTs results in reduced restacking of rGO sheets and improved energy and power density. Cyclic voltammetry (CV) measurements show that the specific capacitance of the micro‐supercapacitor based on rGO–CNT composites is 6.1 mF cm^?2 at 0.01 V s^?1. At a very high scan rate of 50 V s^?1, a specific capacitance of 2.8 mF cm^?2 (stack capacitance of 3.1 F cm^?3) is recorded, which is an unprecedented performance for supercapacitors. The addition of CNT, electrolyte‐accessible and binder‐free microelectrodes, as well as an interdigitated in‐plane design result in a high‐frequency response of the micro‐supercapacitors with resistive‐capacitive time constants as low as 4.8 ms. These characteristics suggest that interdigitated rGO–CNT composite electrodes are promising for on‐chip energy storage application with high power demands.     

Growth of Vertically Aligned Tunable Polyaniline on Graphene/ZrO2 Nanocomposites for Supercapacitor Energy‐Storage Application

In‐situ hydrothermal method is employed to synthesize graphene/zirconium oxide composite from respective precursors graphene oxide and zirconium oxy‐nitrate. In this method, the graphene oxide is reduced itself to graphene and simultaneously metal oxide gets anchor on the graphene sheets. A novel method is also developed for the preparation of vertically aligned tunable polyaniline on the graphene/zirconium oxide nanocomposite, which leads to achieve high surface area (207.1 m² g^?1), high electrical conductivity (70.8 S cm^?1), high specific capacitance (1359.99 Fg^?1 at 1 mV s^?1), and high electrochemical performances as supercapacitor electrode materials. This vertically aligned conducting polymer gets easy contact with electrolyte ions and provides numerous redox active sites during charging and discharging. Moreover, such a simple and low cost assembly approach can be a pioneer for the large‐scale production of various functional architectures for energy storage and conversions.     

Highly Elastic and Conductive N‐Doped Monolithic Graphene Aerogels for Multifunctional Applications

The simple synthesis of ultralow‐density (≈2.32 mg cm^?3) 3D reduced graphene oxide (rGO) aerogels that exhibit high electrical conductivity and excellent compressibility are described herein. Aerogels are synthesized using a combined hydrothermal and thermal annealing method in which hexamethylenetetramine is employed as a reducer, nitrogen source, and graphene dispersion stabilizer. The N‐binding configurations of rGO aerogels increase dramatically, as evidenced by the change in pyridinic‐N/quaternary‐N ratio. The conductivity of this graphene aerogel is ≈11.74 S m^?1 at zero strain, whereas the conductivity at a compressive strain of ≈80% is ≈704.23 S m^?1, which is the largest electrical conductivity reported so far in any 3D sponge‐like low‐density carbon material. In addition, the aerogel has excellent hydrophobicity (with a water contact angle of 137.4°) as well as selective absorption for organic solvents and oils. The compressive modulus (94.5 kPa; ρ ≈ 2.32 mg cm^?3) of the rGO aerogel is higher than that of other carbon‐based aerogels. The physical and chemical properties (such as high conductivity, elasticity, high surface area, open pore structure, and chemical stability) of the aerogel suggest that it is a viable candidate for the use in energy storage, electrodes for fuel cells, photocatalysis, environmental protection, energy absorption, and sensing applications.